The dramatic resurgence of tuberculosis (TB) has been well documented. This rise has been largely attributed to the HIV epidemic. HIV-infected persons have been shown to be at extremely high risk of tuberculosis both through high rates of reactivation of latent infection and through accelerated progression to active disease after primary exposure. While the response to daily therapy in compliant HIV-infected individuals has been generally adequate, mortality while on therapy has been higher than that seen in HIV-negative persons with tuberculosis. Furthermore, adverse drug reaction rates have been high and there are reports of failures and relapses. Questions regarding the appropriate duration of therapy, and the outcomes of therapy of multidrug resistant (MDR) disease are being addressed in ACTG 222/CPCRA 019 and ACTG 238/CPCRA 026. This trial will seek to determine the frequency and extent to which serum concentrations of four standard first line antituberculous agents in HIV-infected subjects fall outside expected ranges (compared with both currently available literature ranges and newly derived expected ranges for this population).